Many motor vehicles include a stop-start (or start-stop) system configured to automatically shut down an engine of the motor vehicle when the vehicle is stationary to reduce the time the engine spends idling, thereby reducing the fuel consumption of the motor vehicle.
Conventional starter motors are only able to operate to restart the vehicle engine when the vehicle is in a neutral gear. Hence, conventional stop-start systems typically operate to shut down the engine of the vehicle when the vehicle is stationary and in a neutral gear.
Modern mild hybrid vehicles often include a Belt Integrated Starter-Generator (BISG), which enables torque to be provided to the engine, to crank and re-start the engine, while the vehicle is moving and is in gear.
The inclusion of the BISG within the motor vehicle enables the stop-start system of the motor vehicle to be extended so that the engine can be stopped while the vehicle is in gear and is moving, with the option of restarting the engine if the driver wants to accelerate the vehicle. This may be referred to as a rolling stop-start operation of the motor vehicle.
A limiting factor for the ability of a motor vehicle to perform a rolling stop-start operation is often the availability of vacuum pressure within a brake booster of the motor vehicle, which is only supplied to the brake booster while the engine is running. As the vehicle is moving during a rolling stop-start operation, it is more likely that a driver of the vehicle will apply and modulate the brakes during the rolling stop-start operation, compared to during a stop-start operation performed while the vehicle is stationary.
It is desirable to ensure that brake booster vacuum pressure is available to assist a driver in braking the vehicle, and hence, stop-start systems are typically configured to restart the engine when the vacuum pressure drops below a desirable level, ending the stop-start operation.